Duterte spokesman Ernesto Abella expressed disappointment that Ms Callamard did not contact the government before her visit, saying "she has sent a clear signal that she is not interested in getting an objective perspective of issues that are the focus of her responsibility".

"The "war on drugs" does not work," Callamard said in her speech, which criticized the use of violence to fight the illicit drug trade.

The Philippine government earlier sent an invitation for Callamard to investigate the alleged human rights violations in the Duterte administration's war on drugs. "The Government was also informed that the trip was not an official visit", Callamard said.

Callamard is an early critic of the Philippine president and has been challenged by Duterte to debate his war on drugs that has left thousands of suspected drug dealers and users dead. He also accused the United Nations special rapporteur of bias, in supposedly sending a signal that "she is not interested in getting an objective perspective on the issues that are the focus of her responsibility".

"This press briefing would be an opportunity for me to introduce briefly my preliminary findings and for the President to offer his own analysis, reply or rebuttal", Callamard said.

Spokesman Abella said she had failed to respond when the government in September formally invited her to visit, which he said suggested she "would not be approaching her review of allegations concerning our country objectively or comprehensively".

"Absolutely, I am committed to continue my dialogue with the government and I am committed to undertake an official visit either by myself or with the special rapporteur to the right to health", Callamard said on Saturday, May 6, on the sidelines of a drug policy forum at the University of the Philippines in Diliman, Quezon City.

He said the Philippine team in Geneva would raise the issue before the United Nations, saying Callamard's actions were unprofessional.

"This is a matter we have asked our representatives at the United Nations to take up with their United Nations counterparts and it is something our delegation in Geneva will certainly be raising during their current visit", Abella said.

"Our commitment to carrying out the President's social justice agenda remains as strong as our commitment to ensuring that our nation is freed from the damage done daily to millions of our people by the proliferation of illegal drugs throughout our country", he added.

She reiterated that she is only visiting to participate in an academic conference on drug-related issues and was not assessing the situation in the country.

At the same time, she praised those who spoke out against Duterte's drug war. She said she communicated with the Permanent Mission of the Philippines via phone, mail, and email until May 4.

According to UN's Office of the High Commissioner for Human Rights (OHCHR), the review will cover the first 10 months of the Duterte administration, as well as the last five years of the previous Aquino administration.

She was speaking on a surprise visit to the Philippine capital.

Callamard is in the Philippines upon the invitation of several non-governmental organizations (NGOs).

"I think that kind of strategy can carry forward a different policy", added the human rights expert, who visited Manila to attend the drug policy forum.

She heard debates between politicians, explanations of government officials and even watched footage of police and military men.